This invention relates to multi-tone printing employing drop-on-demand printing apparatus. More particularly the invention relates to such apparatus for printing on printing element areas on a substrate which is movable relatively to said apparatus and comprises an array of parallel, uniformly spaced channels provided with respective ink ejection nozzles, a liquid supply means common to said channels and electrically operated means for applying pulses of energy to liquid in said channels to effect ink ejection therefrom.
Aspects of such printing apparatus are described for example in U.S. Pat. No. 4,584,590 and commonly assigned U.S. Pat. Nos. 4,887,100; 4,992,808, 5,003,679 and 5,028,936 of which the contents of the latter are herein incorporated by reference.
The types of printing apparatus disclosed in the references quoted are of the kind in which energy pulses are imparted to ink by displacement of wall portions of the respective ink channels. It is known that the human eye can sense sixty-four gradations of greyscale in multi-tone printing. It is even suggested that twice as many gradations can be discriminated. Accordingly, it is one aim of high quality tone printing, including colour printing, to produce a printer capable of printing a number of greyscale tones as near as possible to the discriminating capability of the eye of the viewer.
In U.S. Pat. No. 4,513,299 there is disclosed a single channel, drop-on-demand ink jet printing device in which droplets of ink having different droplet volumes can be deposited on a print medium at a droplet repetition rate just below the resonant frequency of the ink channel. The different droplet volumes are achieved by following a droplet ejection pulse with additional droplet ejection pulses of like magnitude to the initial droplet ejection pulse at a frequency at or near the channel resonant frequency. The additional droplet ejection pulses cause ejection from the ink channel of further drop volumes of substantially the same size as the drop volume emitted from the channel by the initial droplet ejection pulse. In the series of drop volumes thus emitted, the second and subsequent drop volumes are each connected to the preceding drop volume emitted and the drop volumes join together to form an enlarged droplet which is deposited on the print medium. However, in high density array drop-on-demand printers, i.e. arrays of at least two parallel channels per mm, the known method of droplet ejection severely limits, to only a few, the number of droplet volumes which can be added to the droplet volume initially ejected from any particular channel. This number diminishes rapidly with increase of channel density. It follows that the number of greyscale gradations which can be achieved by this known method is limited in the achievable number of different drop volumes which can be deposited at a pixel of the printed image.
U.S. Pat. No. 4,536,097 discloses a piezoelectric printhead having channels defined by a row of parallel piezoelectric strips which can be deformed in direct mode. Each working channel, with its two piezoelectric strips, is separated from the next by a dummy channel filled with elastomer or air and the arrangement is therefore limited as to the channel resolution that can be achieved. The printhead is not capable of producing drops of varying sizes.
GB-A-2 157 623 discloses a method of operating an ink jet apparatus to control dot size. Relatively small ink chambers have separate large piezoelectric transducers communicating with the respective chambers through a mechanical coupling including a foot and a diaphragm. It is proposed that by synchronously exciting either one or a combination of the fluid and mechanical resonant frequencies of the ink jet apparatus and by repeating this operation in an iterative or successive manner in synchronism with the dominant resonant frequency, a plurality of ink droplets can be ejected within a time period permitting the droplets to merge while airborne or upon the recording medium.
It is an object of the present invention, to provide an improved method of greyscale printing which employs a compact drop-on-demand printer having an array of parallel channels which enables printing at high density and with a substantially greater number of greyscale gradations than has been achievable hitherto.